November 13, 2008

More lab disasters

Ψ*Ψ has a new post on undergrad lab disasters, I would like to add few more of my own making, from two decades ago:

In the junk-room of our chemistry department I found an ancient belt-driven vacuum pump that operated on 380V three-phase AC. I brought it from the basement into our freshly-renovated lab, put it on the bench, plugged it in to see if it worked – and it did. Unfortunately the 380V AC wiring must have been wrong in our lab (we never used 380V plug in there before) or the phase order in the pump itself was switched. At any rate, the motor started spinning backward and the pump pumped out its oil from the inlet hole at once – a gallon of black muck that hasn’t been changed for eons. The intlet had a short piece of rubber hose on and it worked like a nozzle – directing the stream of goodness at the high ceiling, right in the middle of the room whence it rained down all over the place. This mishap actually shut down our lab for two weeks as the oil-soaked plaster had to be knocked off down to the brick and concrete in order to make the new plaster stick.

Some months later I was working as a guest student in another lab (in another building, at another school) – and they had a gas-powered water heaters installed above the sinks because their building lacked central hot water. It was a strange and dangerous thing to have in the lab (right next to the organic solvent bottles); and the last person in the lab always made sure to turn off the pilot lights before leaving. One early morning I was washing my hands and water was coming out freezing cold – the pilot light was off – so I grabbed matches and without turning off the running water (and the stream of gas), l lit the pilot. A yellow fireball shot up and the casing flew off from the heater infront of me, with an impressive bang. When my ears stopped ringing I could hear a calm voice from the opposite corner of the lab where a colleague sat at her desk: “Mr. Borivoj, I promise that the next time I’ll pour my coffee over you.”

The same colleague a week later decided to clean up and inventorize all her glassware – she emptied her drawers and put it all on a long bench. Meanwhile I was distilling 0.5L of old and nasty-looking N-methylmorpholine to which I added lots of calcium hydride – and when I finished I had CaH2-rich leftover sludge in the distillation flask. I was asking around if it was OK to quench it with ethanol (I had quenched NaH and BuLi before) and a faculty dude said I should go right ahead; I did not realize they probably did not work much with CaH2 in that lab. So I was carefully adding ethanol with cooling, the bubbles were coming out, calm and nice. One hour later (back from lunch) I added some more ethanol – no bubbling anymore – so I was certain I could pour that into the waste, and I went to the sink to dilute the sludge with water a bit so that I could pour it out. Suddenly it became clear that ethanol does not really quench CaH2, and water does. The mud volcano in my hands erupted away, spewing the hot lime and fishy amine on the nearest bench – all over that clean and inventoried glassware. The owner again took it calmly – she just muttered “We have to assign you a working space in the hallway”…

Two giant rotary vacuum pumps fresh out of their crates filled to the brim, not to sight glass lines, with oil. Pop the switch to pump down a large multiple dewar of liquid helium to its lambda point through very large bore plumbing.

Physical chemistry, new faculty, first year grad students. Where there is no old fart there will be disaster.

Yeah whenever I quench CaH2 I make sure to dilute the leftover mud with (wet) toluene, roughly to fill 40% of the flask. Then I quench with some MeOH and then water. Of course, there seems to be an induction period, which I never quite respect enough, but at least the PhMe can take care of a fair bit of the heat being produced. Some people would use heptane, but i prefer the smell of toluene… 😛

Quenching reactions seems to be something of a fine art.
I think everyone has their own favourite way to kill LiAlH4. I vaguely remember a J. Org. Chem. paper that discussed this in great detail, giving a sand-like residue by adding NaOH versus the sticky goo that can often result.

OMG, no – but I heard horror stories from other places. (Our NMR man is my close friend and a kind person but I think he would kung-fu me, for a much lesser offense than a quenched magnet. Whenever I was helping him with filling liquid He and few other minor things – when he needed a second pair of hands – he has been pretty particular about every little detail.)

LAH quench – the JOC recipe is from Fieser if I remember correctly and it is for each gram LAH used, first 1 mL of water dropwise very slowly, then 3 mL of 15% NaOH gradually, then 1mL of water again and stirr for 30 min.

And if you want the salts to be even more insoluble (because they appear to be partially soluble) in THF, after finished quenching, pour the entire suspension into 2X the volume of Et2O and stir 1h. Works like a charm every time for me.

Yeah, CaH2 can be pretty nasty to quench if you don’t respect the induction. Acetone most certainly will not work to do the quench. Just think of the pKa difference between acetone and even normal alkoxides. I don’t fully understand why it is that alcohols won’t kill it completely since the pKa differences between water and alcohols are ~2 units, but I suppose that’s sufficient to make all the difference.